Abstrakt

In the 21st century Gdansk had been affected by two rainfall events with a total precipitation amount of over 120 mm each. Rainfall parameters exceeded the regional values of the rain occurring once every 100 years. At the meteorological station of Gdansk University of Technology, July 9th 2001 was recorded 123.5 mm and July 14th 2016, 150.8 mm rainfall (within 24 hours observation period). Despite the same seasons of summer months and a duration of about 16 hours, both rains were characterised by different course and causes of their occurrence. The study compared the causes, course and consequences of these extraordinary rainfall events. In the paper the precipitation parameters were compared with the depth-duration-frequency curve (DDF curve) which were determined on the basis of actual rainfall events that occurred in the 20th century in the northern Poland. It was found that the analysed rainfalls significantly exceed the theoretical dependence, primarily due to the total amount of precipitation, which accounted for 190% of normal rain in July as well as due to the duration of the phase of the most intense rain (up to 8 hours in year 2016). The two mentioned rainfall episodes were also compared with other rain indicators. One of the rain indicators used in Poland is Chomicz's rainfall intensity 12 grade scale. This scale classifies rainfall episode as normal rain, storm, heavy storm or torrential rain. Both events are partially classified as torrential rain with grade U5. It is worth noting that the losses after the flood of 2016 were tallied many times lower than after the rainfall event in 2001. It was possible due to massive investment in storage reservoirs on both the natural water and storm sewer systems. Development of the land surface in Gdansk also takes into account the need to better retention of rainwater. It should be noted that the above two events significantly exceeded the parameters of the precipitation that occurs once in 100 years. The mentioned rainfalls occurred at an interval of only 15 years. It seems to be a proof that the climate changes must be taken into account when estimating the size of precipitation for current and future activities.

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